Process for recovering flotation reagent



May 17, 1960 w. P. wlLsoN ET AL 2,936,887

PROCESS FOR RESOVERING FLOTATION REAGENT Filed Sept. 27, 1957 72 Mfer 44 if @neen #47% @U9/6: l? 4 6e s4 i 4 46 56 /ffffref d f @Mm-f 167/16. L.

United States PatentO PROCESS FOR RECOVERING FLGTATION REAGENT Application September 27, 1957, Serial No. k686,748 4 Claims. (Cl. 209-166) This invention relates to processes for separating` the components of a particulate mineral mixture in an aqueous medium by virture of dierential action produced with the aid of one or more reagents. Such reagents adhere differentially to the respective components of the mixture and thereby modify their performance in the medium.

A primary object of the invention is to provide a convenient and economical process for removing such reagents from the particles of the mixture after the desired separation has been achieved.

A further object of the invention is to permitrecovery of the reagent in such form that it can be reusedgas for conditioning and separating another charge of the mineral 'i mixture.

The invention further provides overall cyclic processes which areparticularly eifective, convenient and economi Patented May 17, 1960 table agglomeration, in which the conditioned aqueous slurry is subjected to a shaking motion on an obliquely rilfled, slightly sloping table having such an angle of slope that the heavier uncoated granules tend to be retained behind the riflles while the coated granules tend to wash over the riffles. By means of suitable diversion gates, the discharge may be collected as two or more separate products, at least one of which is appreciably concentrated with respect to the desired product. In actual practice, separa'- tion of a granular mixture after conditioning with a reagent -may involve a relatively complex series of steps, sometimes including several successive flotation steps or including lboth flotation and table agglomeration or jigging and other mechanical means. Conditioning agents for' any such processes will bereferred to for convenience as otation reagents. v

Separation of the components of the initial mixture by such procedures may be virtually complete, or may bey only partial. In many instances the initial mixture typical-` ly includes some particles which contain both the desired and the undesired components ofthe mixture,` so that the gravityseparation in aqueous medium cannot, in practice, be complete. The term separatiom as employed in the present specification and claims, includes partial as well as complete separation of ltwo or more components, and thus includes operations more commonly referred to as i concentration Amine flotation reagents are used in the described manner for separating the components of particulate mineral cal and which have additional advantages over previously i available procedures, as will appear, particularly when one or more of the components of the mineral mixture are water soluble. I

The invention relates more particularly to cationic reagents which include an amine group, and to other reagents such as kerosene and other oils which are commonly used to supplement their action. Typical amine reagents comprise salts of primaryr alkyl amines in which the alkyl radicals are straight chains containing from 8 to 2.2 carbon atoms. Such alkyl amines, which may be fully saturated or partially unsaturated, are usually prepared from natural mixtures of fatty acids, and their exact composition depends upon the origin of those materials. Illustrative of such reagents are those derived from tallow fatty acids, typically comprising both saturated and unsaturated chains of 16 or 18 carbon atoms; those derived from soya fatty acids, typically containing a greater proportion of unsaturated groups than the tallow amines; and those rkderived frorriY coconut oilnfatty acids, comprising a more diverse mixture ofchainlengths extending down to 8 carbon atoms and typically substantially saturated. j Y Y Q Salts of such amines, for example their acetates vor hydrochlorides, ionize in water, andthe long chain cation adheres selectively to mineral surfaces which are characterized by negative surface charges. The resulting surface coating of amine reagent, suppleivnented` in some instances by one or moresecondary reagents, is hydrophobic. `With suitable conditioning of a mineral mixture, air bubbles accumulate on vthe reagent-coated surfaces, reducing the effective specifigravity of the coated par? ticles relative to that of the uncoated components of the mixture.

The actual separation of the conditioned'mineral mixture may be accomplished by any of a variety ofY knownI operations. Such separation may be accomplished, for

example, by an actual ilotation process in .water 'or-satumixtures of many different types, the primary requirement being that the reagent will absorb selectively on surfaces of at least one mineral component of the mixture. Illustrative instances of such selective adsorption are the coating of feldspar surfaces in preference to silica, permitting separation ofa particulate mixture of those minerals; the coating of silica in preference to phosphate rock, whereby the phosphate is typically recovered as the sink fraction; the.v coating of vermiculite in preference to particles of silicate or silica; the coating of mica and silica in preference to beryl ores; and the coating of potassium chloride in preference to sodium chloride.

For the sake of clarity and deiiniteness, the invention will be illustratively described primarily with specific reference to the use of amine flotation reagents for the latter purpose, as exemplified in obtaining a potassium concentrate from sylvinite ore. Long ,chain alkyl amine salts are particularly useful as flotation reagents for that purpose, tallow amine acetate and tallow amine hydrochloride being illustrative. Such reagents adhere selectively to potassium chloride surfaces and form thereon a coat-y ing that is not wet by water. Surfaces of sodium chloride are not so coated 4by the amine reagent. With suitable conditioning treatment, which typically includes agita-y tion of an aqueous slurry of properly sized and scrubbed `sylvinite ore in the presence of the flotation reagent, air bubbles form on the coated potassium chloride surfaces, reducing the effective specific gravity of the potassiumrich particles of the mixture. The potassium-poor particles, which consist primarily of sodium chloride, YretainV substantially their normal specific gravity in the aqueous medium. y Y

`For conditioning sylvinite ore, especially when it lincludes particles coarser than about 20 mesh, Tyler Standard, Yit is usually desirable to use a ilotation reagent comprising in combination a suitable amine reagent and an oil, such as crude oil or diesel oil. The oil is generally believed to be adsorbed on the layer of amine reagent on the potassium chloride surfaces,vthereby forming a more complete and effective water repelling Icoating on the potassium-rich granules. The combination of an alkyl amine 'salt and oil is illustrative of the manner in which :onechemical subiancemay be employed to sup" plement the action of another. Such a combination of distinct substances may be considered to constitute a flotation reagent.

-In previous practice, -a disadvantage of all aqueous separation process of the typedescribed has been the diculty in separating the adhered ilotation reagent from the particles of the separated product. That disadvantage has been particularly serious in the present illustrative instance in Which sylvite is separated from the salt component of sylvinite ore with the aid of an amine otation reagent which may include oil. When the separated product from such a process has been dried at temperatures of 250 to 350 F., for example, the residual amines on the particles tend to be converted to amides, or other organic` compounds which have a higher boiling point. When such organic compounds and the crude oil, if any, distill ofIl` the particles during normal drying, they tend to condense on surrounding surfaces, such as the walls of chutes and stacks. Although the total concentration offlotation reagent in the product is usually very small, typically less than 0.1%, such deposits accumulate steadily and eventually form a potential lire hazard. Furthermore, the deposit remaining on the particles tends to blind the openings in screens used to remove the nes from the concentrated product.`

Those diculties may be overcome by heating the product to about 500 to 550 F., at Which temperature the more volatile oil and other compounds are carried off and the remaining tarry residue is charred to a sooty coating. Although the fire hazard and screen blinding Since the resulting mineral product is substantially free of iiotation reagent, it can be dried at normal drying temperatures, such as 250 to 300 F., Without creating inconvenient deposits on adjacent surfaces. Conventional screening of the dry product, as to remove undesired rines, can be carried out without dihiculty. Moreover, if it -is desired for any reason to heat the product to higher temperatures, such as 500 to 550 F., that can be done Without discoloring the product or causing undesirable dustiness.

In accordance with a further `aspect of the invention, removal ofthe coating of flotation reagent facilitates Washing of the product. That is of particular importance whenthe mineral product may be contaminated with a material that is soluble in a suitable aqueous medium. In the illustrative instance of producing a potassium concentrate from sylvinite ore, the concentrated product is Vusua-lly'contarninated with sodium chloride, much of which is present in particles which consist primarily of potassium chloride. By Washing such products with water -or with a solution not saturated with the contaminating salt, it has previously been possible to remove a p small proportion of the contaminant. We have now disare thereby eliminated, the resulting dark color of the Y product may not be desirable. Moreover, the sooty coating releases dust at each handling of the product. '.I'hatV dusty condition is very persistent since it is not removable by screening.

In accordance with the present invention, all such disadvantages are avoided economically and conveniently by removing the flotation reagent from the separated mineral product while the latter is Wet. That is accomplished by treating the coated particles with an alkaline aqueous solution. We have discovered that such treatment causes the otation reagent to release from the particle surfaces. The released reagent may then be removed With the aqueous medium from the solid particles. The solid concentrate thus obtained is substantially free of reagent, andmay be dried and further processed in any desired manner Without difficulty.

The alkaline solution for thus treating the adhered flotation reagent may be provided conveniently by adding a suitable quantity of an alkali metal or alkaline earth metal hydroxide, either in solid form or as an -aqueous solution, directly to the Wet mineral product as it cornes from the aqueous separation process. For example, so` diumor potassium hydroxide or calcium hydroxidemay be employed for that purpose. It is preferred in general to employ, if possible, a hydroxide of a metal already present in the system. Before adding the alkali, the slurry resulting from the aqueous separation processv is preferably partially de-watered so that it forms a relatively dense slurry or pulp, comprising typically approximately 80% solids and 20% aqueous medium. After addition of the alkaline reagent, the mixture is thoroughly mixed in any suitableV manner to carry the reaction to completion. The reaction may be accelerated, if desired, by heating moderately, for example to about 180 to 220 F. The released otation reagent, such, for example, as an amine compound or a combination of amine and oil, is typically Water insoluble and forms a suspension in the aqueous component of the mixture. That suspension is readily rinsed away from themineral product, as by Washing with additional aqueous medium. That medium is preferably alkaline but may be substantially neutral. Hence the mineral product can be separated from the suspension of reagent and aqueous medium-in any suitable type of washing and de-watering equipment.

covered that the effectiveness of such vWashing of the product can be greatly increased by removing the flotation reagent before carrying out the Washing step. The reason for the improved effectiveness `is believed tobe that. the coating o-f flotation reagent, `although it adheres preferentially to potassium chloride surfaces, tends to vshield an appreciable part o-f the sodium chloride surfaces also, particularly when the latter comprise small areas nearly surrounded by potassium chloride.

Because of the greater potential utility of a Washing operation when carried `out on a concentrated product that is free of otation reagent, the detailed manner of carrying out that Washing operation acquires Vincreased importance. It has previously been recommended that potassium concentrates from sylvinite ore should be further. concentrated by washing with a solution that is substantially free of sodium chloride and is saturated With potassium chloride. We have discovered that improved results are obtained When the Washing solution is initially `only partially saturated with potassium chloride. Such a. partially-saturated solution has the advantage that it dissolves the sodium chloride contamination ymore rapid- `1y andis capable of 'dissolving a larger concentration of sodium chloride before becoming saturated. And, perhaps of even greater importance, if the Washing solution is initially saturatedwith potassium chloride, we have found that, as. sodium chloride is taken up by the solution, the latter rapidly becomes supersaturated with respect to potassium chloride. Very fine crystals of potassium chloride are thereby precipitated. Such tine precipitate is often troublesome to handle, and, at best, results in a lfine Vdust in the iinal product, which must be removed as by screening. When washing is performed with a solution that is initially only partially saturated with potassium chloride, the washing solution does not become superrsaturated, and the described ditliculties are thus completely avoided. The most effective results are obtained with awashing solution that contains initially Iless than about 10% of saturating concentration of sodium chloridey and between about 30% and about 70% of` saturating concentration of potassium chloride;

in-accordancewith a further aspect of the invention, we have discovered that the reagent, after its separation asiasuspension in aqueous-'medium Yfrom the solid concentrate, can belrecovered and itsutility as a reagent re-` stored. Such reactivationofthe' recovered alkaline revagentcan be. accomplished by neutralizing it with any suitable acid. That neutralization restores to the reagent itsl initial property of adhering selectively to one, component of the mineral mixture. that is being separated. Suchqneutralization of the released reagent, which is typically'inr theform of asuspension in alkaline solution,

may be accomplished by 'addition of acid of any 'convenient type. That acid may, for example,v be an inorganic acid such as hydrochloric acid, phosphoric acid or sulfuric acid, or may be an organic acid such'as acetic acid or citric acid. It is ordinarily preferably to employ an acid of which the anion is alreadypresent in the system. VSufficient acid is added .to substantially neutralize the mixture of released reagentand aqueous suspendingV medium, that is to reduce the pH of that medium to approximately 7.2 to 7.8.

Thus, in the present illustrative operation, the tendency of the amine flotation reagent to adhere selectively to potassium chloride and not to sodium chloride is destroyed by treatment with hydroxide, and is'restored by .neutralization with acid. The invention thus presents for the first time the possibility of re-using the same flotation reagent in sucessive cycles of operation, leading to great economy in the overall system. That advantage is obtainable in addition to the marked improvement in the inalY product, already described. i

It has been found desirable, in order to obtain maximux'nV efficiency of the reactivated reagent when oil is present, to perform the described acid .treatment only when the released reagent is actually or substantially in the presence of the fresh charge of mineral mixture that is to'be conditioned. If the alkaline oil-amine suspen-l sion of reagent is neutralized while isolated from that fresh mixture, the reactivated reagent tends toV form thick greasy curds, which only become further condensed uponapplication of heat. It is believed that such behaviorv is due to the tendency of the reactivated reagent to adsorb on itself. However, when the neutralization is performed in the presence of afresh chargeof mineral mixture, the reactivated agent adsorbs in a normal'manner on one component of the mixture and effectively vperforms its intended function. The small proportion of reagent that is lost during each cycle of operation can readily be made up by suitable addition of fresh vr'eagent'to the conditioning apparatus. f

A full understanding of the invention and of its further objects and advantages will be .had from thek following description of certain illustrative systems by which it may be carried out. Those descriptions, and the accompanying drawings which form a part of them, are intended for illustration only and not as a limitation upon the scope of the invention, which is` defined in the appended claims.

A In 4the drawings: Fig.A 1 is av schematic drawing representing an illustrative system by which sylvinite ore may be treated in accordance with the invention; andl Fig: 2.isV a schematic drawing illustrating a modified system.

As represented in Fig. 1, sylvinite ore is introduced at 10. Such ore consists primarily of potassium chloride,

` sodium chloride, and a relatively small proportion'of insoluble clay. The ore at may be consideredto have been'suitably sized, as by crushing and screening,to a particle size that will typically pass an. 8 mesh screen and be held on a2() mesh screen. Most of the clay in the ore is released' by scrubbing inV a vsaturated. brine, 'supplied at Y11, in apparatus of any suitable type, indicated schematically at 12. The released clay is removed withl most of the brine, as by the de-brining screw-indicated schematically at 14.

The ore is then'introduced into the conditioner 20,

urated brine, causing the reagent to adsorb on" the surfaces of potassium chloride. Air bubbles produced by the agitation form on the amine and oil surfacesin the manner already described.

The conditioned ore is then transferred at v23 to a riifled, slightly sloping table, indicated schematically at 24, where it is subjected to a shaking motion. The uncoated sodium chloride tends to retain behind the riiiles and be washed along the length of the table, while the particles of potassium chloride, with their coating of amine and oil, tend to wash over thek rifes. By means of suitableV diversion gates the discharge is typically collected in three products: (l) concentrated potassium chloride at 2S, typically containing an amount of potassium corre-` sponding to more than 6,0% K2O; (2) middlings at 26,y

containingthe ner sized particles of potassium chloride and the coarser sized particles of sodium chloride, as well as particles which contain mixed sodium chloride and potassiumchloride; and (3) tailings at 27, consisting primarily of sodium chlorideand containing typically about 2% KZO.

The middling product at 26 may be further refined, as by treatment in a battery of flotation cells, indicated schematically at 30, usually after addition of a small amount of frothingagent, such, for example, as isocarbinol supplied from a source indicated at 31 via the rvalve 33. Such flotation treatment typically divides the mixture into a oat concentrate at 32, containing typically about 59% KZO, and sinkv tailings at 34, which may contain as much as 4% K2O. The tailings at 34 are combined with the tailings at 27 from the tabling operation and sent to a storage ypile at 36, or otherwise dis- Y posed of. The flotation product at 32 is combined with the'potassium-rich product at 25 fromthe tabling operation.V In previous practice that product at 34 was .de-

watered and dried to produce the iinal product ofthe` operation. That product suffered kin one respectv or another 'from the presence of residual flotation reagent, a has been described.

In accordance with the present invention the combined Vproduct at 34 is preferably lde-Watered, as'in the deheavy slurry in mixer 40, as by adding alkali metalf'or alkaline earth metal hydroxide as a concentrated aqueous solution directly to the mixer, as from a suitable storage container 44. The amount of alkali addedfis controlled,

as by the valve 45, and is sufcient to cause release of 55.-.-substantially all of the amine reagent reaching mixer 40 with the concentrated potashrpr'oduct 34.

z; The most effective and most economical amount of alkali for any particular conditions 'of operation, type of reagent is released from the ore.

one half to about ten pounds of sodium or potassium hyore, and detailed otation conditioning of the ore can readily be determined by trial, the amount of alkali being gradually increased until substantially all of the otaton Usually, from about droxide per ton of ore concentrate will be found to give satisfactory results, an appreciably smaller quantity being required when the treatment is carried out at moderately elevated temperature, such as' 150 to 1903117., yrather than at room temperature. As a specific example, with typical sylvinite from the beds at Carlsbad, Newv Mexico, which has been conditioned by addition o f one pound of tallow amine acetate and four pounds of crude oil per ton of ore, satisfactory release of the flotation reagent from the potassiumA concentrate jhas been obtained by addition of lfourpounds of sodium hydroxide per ton of concentrate atmixer 40 at room temperatureor.by one pound of soa dium hydroxide when the mixture is treatedy at 190 I". After treatment with alkaline brine solution in mixer. 40 the potassium concentrate is rinsedby brine supplied via the line 46. The solid particles are thereby partially freed from the flotation reagent, which typically forms a suspension in the aqueous phase of the slurry. The mixture may then be delivered via the line 48 to suitable apparatus, shown schematically as the centrifuge Si), for further Washing and de-watering. The potassium concentrate emerges from centrifuge i) substantially free of flotation reagent and may be delivered directly to a drying apparatus of any suitable type, as represented at SZ. The dried product is preferably screened, as at 54, to remove any undesired lines, Vand is typically then delivered ready for shipment at 56. The separated lines 58 may be disposed of in any desired manner. A particularly effective way of utilizing such lines is to dissolve them, as in a dissolver of conventional type indicated at 60, in waterwhich is supplied at 62 to make upa deiicitof water removed from the system. That water is supplied via control means such as the valve 63 at such a rate that the resulting solution contains a concentration of potassium chloride corresponding to approximately 50% saturation. That solution, which typically is less than saturated with respect to sodium chloride, is then supplied, as by the pump 64, to line 46|, and is used as washing brine in mixer 49. A portion of the same solution may be supplied via the line 47 as washing brine in centrifuge Sil. Additional plant brine may be' supplied to mixer 40 and to centrifuge 50 if required, as indicated by the line 66.v As has already been indicated, we have discovered that the utilization of` a solution of the described type for washing the potassium concentrate provides a marked irnprovement in the degree by which the potassium concentration'of that product can be increased. For example, in the described type of operation, the release of reagent from the potassium concentrate followed by washing of the concentrate with brine of the described type has been found to result in an overall increase in the recovery of potassium from the ore of approximately 3% as compared with the best available previous practice.

The released reagent is carried as a suspension in the alkaline brine from mixer 4t) at 68 and from centrifuge 50 at V69. That reagent suspension may be delivered via the surge tank 70 by a pump 72 to the return line 74. In preferred form of the system, a mixture of that alkaline brine and released reagent is supplied to conditioner 20, together with a suitable quantity of neutralizing acid,

which may be admitted from a storage container 76 viav suitable valve means 77. Acid is thus supplied to conditioner in suiiicient quantity to reduce the pH of the alkalineA brine and its contained dotation reagent to approximately 7.2 to 7 .8'. That is typically accomplished by an amount of acid approximately equivalent to the alkali added at 45. With the described procedure, that neutralization takes place in the presence of granular ore in troduced from de-brining screw 14. Under those conditions the neutralized reagent is found to react satisfactorily with the fresh charge of ore even when an appreciable amount of oil is present, selectively adhering in normal fashion to the potassium surfaces of the ore. Additional reagent may be supplied from a source Y80 via valve 82 to conditioner 2t) to make up any loss of reagent which may occur in the described circuit. That reagent circuit includes movement from conditioner 20 to mixer liii-asia coating on the-potassium surfaces, and movement from mixer 40 back `to conditioner 2i) in the form of an inactivated suspension in an alkaline brine.

Fig. 2 represents schematically a modiiied system which illustrates the wide variety of different manners in which the released. reagent may be handled. For clarity of illustration, the portion of the system corresponding to the upper part. of Fig. l is omitted in Fig. 2. Potassium concentrate: is. delivered at. 34. from. a concentrating, plant 8 whichmay utilize `any suitable type of aqueous process',v for example that described. and shown in the upperfpart of Fig. 1. That concentrate, with its coatingof reagent,I

is. de-watered at 36 and processed in mixer 40, centrifuge 50, and dryer 52 in the manner already described in con nection with Fig. 1.

The dry potassium concentrateobtained at 80 may then be further processed as may be required. The reagent released from the potassium surfaces in mixer 40 as a result of caustic treatment is obtained as a suspension in an alkaline brine at 68 and 69, as already described. In the present system that Valkaline brine is delivered to a thickening tank 94. The reagent is removed by skimming-at 96 and the alkaline brine is recovered as a substantially clear solution by bottom withdrawal at 9S. The relatively dense mixture of reagent and brine at 96 may then be discardedif desired, or, for example, may be re-used with suitable acidy treatment in the manner already def scribed in connection with Fig. 1. f

The clean alkaline brine at 98 may be' divided, as by'a control valve 99, into two portions. One of'those portions is supplied viathe line 100 to line 46a, and is used as washing solution in mixer 40 and centrifuge 50, where it may supplement `or replace washing solutions of the type described in connection with Fig. 1. It is noted particularly that the alkaline condition Iof the brine so obtained is desirable, since it tends to reduce the alkalineeded from 44 and insures maintenance of the inactivated condition of the released reagent while the latter is beingV separated from the potassium concentrate. The other portion of the clean brine at 98, which typically comprises any excess over that required for the described washing operation, may be directed by valve 99 to the line 104 and the tank 106. Such Ybrine may be neutralized in tank 106, as by addition of a suitable quantity of acid froma supply 108 via a control valve 109. The resulting neutralized brine may be returned via the line 110 for general use in the plant at any desired location, for example at 11 of Fig. 1. In typical practical operation of a plant of the described type, thebrine requiringsuch neutralization, if any, typically'comprises only about 10% of the total clean brine delivered lat 98 from thickener 94. Hence the recirculation of washing brine from 98 to line 46a without neutralization permits a great reduction in the amount of acid consumed.

Itwill be understood that a thickener such as 94 may, if desired, be introduced into line 74 of Fig. 1, and part or all the resulting clean alkaline brine may then be combined with washing brine from pump 64 of Fig. 1 in any proportions desired. Many other changes may be made in particulars of the systems that have been described, which systems are intended only for illustration and not as a limitation uponv the scope of the invention.

We claim; p

1. A dotation process for separating potassium chloride from ore containing potassium chloride, sodium chloride and insoluble clay, which Acomprises pulping said ore in an aqueous brine, adding to said pulped ore a flotation reagent which selectively adheres to and coats the potassium chloride particles in said ore, said reagent cornprising an alkyl amine salt having an alkyl radical of from about 8 to about 2.2 carbon atoms andan oil, separating the reagent coated *potassiumV chloride particles from the uncoated portions of said ore, contactingthe reagent coated potassium chloride particles Vwith an aqueous alkaline solution whereby the reagent is separated from the potassium chloride and recovering the resultant potassium chloride/from the separated reagent.

2. The process of claim 1 which includes thejsteps of:ad'mixing tlieseparated `iiotation reagent with additional potash ore and neutralizing said admixture whereby the flotation reagent selectively adheres to the potassium chloride in said admixture.`

3. VA process for separating potassium-chloride from ore containing potassium chloride, sodium. i chlorideA Vand insoluble clay, which comprises crushing said ore, pulp ing the crushed ore in an aqueous brine, separating the pulped ore from the insolube clay, treating the separated ore with a flotation reagent which selectively adheres to and coats the potassium chloride particles in said ore, said reagent comprising an alkyl amine salt having an alkyl radical of from about 8 to about 22 carbon atoms and an oil, subjecting the treated ore to a otation treatment, recovering the reagent coated potassium chloride from said otation treatment, contacting the coated potassium chloride particles with an aqueous alkaline solution whereby the reagent is separated from the potassium chloride particles and recoveringV the potassium chloride particles from the separated reagent.

4. The method of separating potassium chloride particles from an adhering coating of a otation reagent comprising an alkyl amine salt having an alkyl radical of from about 8 to about 22 carbon atoms and an oil, which comprises slurrying the coated potassium chloride particles with an aqueous alkaline solution containing a material selected from the group consisting of alkali metal hydroxides and alkaline earth metal hydroxides, whereby the otation reagent is released from said particles of potassium chloride and separating the resulting substantially uncoated potassium chloride from the released otation reagent. Y

References Cited in the file of this patent UNITED STATES PATENTS 

